This is an extract from the conclusions of a scientific evaluation of bicycle helmet design and effectiveness: In case any bicycle helmet designers or Australian politicians are reading this I will do my best to explain the physics of angular versus linear acceleration/deceleration for them.
"Designing helmets to reduce linear acceleration suits the helmets industry which has, in
effect, made a huge investment in the theory that it is the main cause of brain injury. Because the theory is widely accepted, claims that helmets prevent injury or even save lives are plausible enough to persuade the public to buy them and politicians to pass laws to compel their use, creating an assured market for them. Finding practicable means to reduce angular acceleration is an unsolved problem, however; there is no money in it for industry."http://www.cyclehelmets.org/papers/p787.pdf
In a human there is a big heavy bit called a body and a lighter bit on top called the head. Joining the two is a wobbly flexible bit called a neck. The cyclist is traveling through the air at high speed but eventually he will hit something that will slow him and his head down very quickly:
If the head slows down by rotating suddenly its is called angular acceleration and will cause the brain to rotate in the skull causing blood vessels to and nerves all around the skull to tear.
But if the head does not rotate the deceleration is called linear acceleration and generally causes brain the brain to bruise and bleed internally at the point of impact.
The human brain is good best at surviving linear acceleration/deceleration perhaps because it is what happens with most loss of balance related head impacts. However the brain is far less able to withstand angular accelerations and these are commonly associated with higher speed impacts like those when being thrown from a bicycle.
You can test this with a football by kicking it hard against a hard object or surface:
If the ball does not rotate it has been subject to linear deceleration like those that current cycle helmets are designed to cope with.
If the impact of the ball cases it to rotate it has been subject to angular acceleration on impact, where the non-rotating ball suddenly accelerates rotationally. Angular acceleration forces are usually more violent for a given impact speed and current helmet designs/standards are not designed to cope with this.Now go our and kick some footballs onto a flat road surface to see if they rotate as they land. Alternatively, use your car, HGV or one belonging to a friendly neighbour as a target.
9/16" Suntour BMX MP-1000 pedals / 56 tooth TA 6-bolt chainring.(I'm not bothered if the teeth are worn or damaged)